Mysterious MFA was asked for when user is trying to use Bing search

When user tries to use Bing search(Input the key words and etc), mysterious MFA was prompt and the user wants to know where this MFA came from.

 

As we confirmed the access logs of the user, we found that an authentication request to Azure AD with hint "amr_values=mfa", this is what triggers MFA.

 

As far as we know, this authentication request came from an web app registered as Microsoft's 1st party web app called Bing.

Does anybody know why the hint is added in the authentication request to Azure AD?